Network providers have many options for installing cables within a network. For instance, cables can be visible, as from pole to pole in an aerial installation, or hidden from sight, such as in ducts or directly buried. Many locations prefer or require that cables are underground so that the poles and cables do not distract from the scenery. Moreover, there is a generally a lower maintenance cost for underground cables. Consequently, many communication and power companies locate cables underground.
Accurately locating and identifying installed cables is important such as when construction takes place close to the cable or when the cable requires entering for routine maintenance or repair. Often times, records and “as-built” maps of cable networks are not sufficiently accurate for locating and identifying the cables. One drawback for cables installed underground is locating and identifying them after installation because they are not visible. Additionally, locating and identifying cables can also be difficult with aerial installations. More specifically, locating an aerial cable may be relatively easy since it is visible, but there is still the question as to whether it is the desired cable.
The most commonly used method for locating underground cables, commonly called toning, is to induce a electromagnetic signal onto a metallic element in, or associated with, the underground cable at an access point such as a pedestal or closure. In other words, the cables have a metallic wire dedicated for toning either as part of the cable or buried in close proximity to the cable. A handheld antenna having a receiver unit is used for following the path of the cable indicated by the continuous path of the metallic wire. If the metallic element is not grounded, the path of the cable can be followed up to several kilometers from the point of signal induction. However, this method requires a metallic element within the cable or located in close proximity to the cable. Metallic based cables such as twisted pair communications cables or coax cables by their nature have a metallic element, however, many cables do not have a suitable element for sending a toning signal. By way of example, many fiber optic cables are dielectric designs that advantageously exclude metallic elements, thereby avoiding the issues associated with lightning strikes and grounding the cable; however, these dielectric cables are not tonable. Moreover, many communication companies prefer dielectric cables if possible for avoiding issues associated with lightning strikes and grounding the cable.
Because of the need for rodent protection, many direct buried cables use metal armor, which may allow toning. However, expecting that metallic armor will work as a toning element is problematic, since a toning element needs to have good electrical insulation from the earth ground. A common damage in lightning prone areas is that current surges from lightning burns through the jacket of an armored cable at regular intervals and the armor becomes grounded. In many cases, there is no damage to the cable transmission characteristics, but the armor becomes grounded and a toning signal will not travel in the armor past the grounded point. Like the armor cables, lightning damage to the jacket over the toning wire can cause grounding and significantly inhibit the toning capacity for locating the cable. Whereas, burying a wire in close proximity to the cable will have similar issues such as becoming grounded or completely severed by lightning strikes, thereby inhibiting toning capability. Thus, other methods for detecting cables were developed.
For instance, U.S. Pat. No. 5,106,175 discloses a method for using electronically resonant markers for locating and identifying a cable. The electronically resonant markers receive incident electromagnetic radiation at a first frequency and second frequency and re-radiates at an intermodulation frequency (a function of the first and second frequencies) that is different from the two incident frequencies. The electronically resonant markers comprise a foil circuit disposed on a planar surface of a relatively thin dielectric substrate along with a surface mounted diode. The shape of the foil pattern and the arrangement of portions thereof on the planar surface of the dielectric substrate provide capacitive and inductive elements. Further, the diode acts as a signal mixer in the foil circuit for creating the intermodulation frequency from the incident first and second frequencies. This foil circuit structure is relatively complex and adds significant cost to the cable when using enough antenna elements for locating and identifying cables. The present invention solves the shortcomings of the prior methods for locating cables.